Oligomerization is generally understood as meaning the reaction of unsaturated hydrocarbons with themselves to form correspondingly longer-chain hydrocarbons, the so-called oligomers. Thus, for example, an olefin having eight carbon atoms (octene) can be formed by oligomerization of two olefins having four carbon atoms (butene). The oligomerization of two molecules with one another is also referred to as dimerization.
The resulting oligomers are intermediates that are used, for example, for producing aldehydes, carboxylic acids and alcohols. The oligomerization of olefins is carried out on a large industrial scale either in the homogeneous phase using a dissolved catalyst or heterogeneously over a solid catalyst, or else with a biphasic catalyst system.
In the case of the heterogeneously catalysed processes, oligomerization over acidic oligomerization catalysts has long been known. Systems employed industrially include acidic catalysts, for example zeolites or phosphoric acid, on a support. Isomeric mixtures of more or less branched olefins are obtained here. The designation acidic catalysis or acidic catalysts here describes BrØnsted acidity, i.e. the catalyst provides catalytically active protons. Often employed for non-acidic, heterogeneously catalyzed oligomerization of olefins with high dimer selectivity in the art are nickel compounds on support materials, wherein the nickel does not provide protons but rather acts as an electron pair acceptor (Lewis acid). Thus WO 95/14647 A1 describes a nickel catalyst comprising a support material consisting of the components titanium oxide and/or zirconium oxide, silicon oxide and optionally aluminium oxide for olefin oligomerization. Over these catalysts, mixtures of linear butenes are oligomerized to C8-olefins with a selectivity of below 75%. It is thought that the catalytic activity of nickel-based, heterogeneous catalysts for oligomerization of olefins is based on the interaction between nickel cations and surface aluminium atoms.
In the case of the oligomerization there are various mechanisms by which the oligomerization may proceed. These include acidic catalysis where the olefins form with the acid centre of a catalyst a carbenium ion which can react with the double bond of a further olefin, thus forming a new C—C bond. Since the carbenium ion is best stabilized at the most highly branched point of the cation, highly branched oligomers which are relevant almost exclusively for the production of fuels are formed. Oligomers having a relatively high linearity in particular are required industrially for further processing to afford chemical end products such as plasticizers or surfactants. A further mechanism is the coordinative mechanism where the first olefin bonds to the catalyst coordinatively. A further olefin can become attached there and lead to the formation of a new C—C bond and thus to the formation of an oligomer. The products of this mechanism are typically less highly branched.